Moves balancing code out of Manager class

[keith-turner]

The balancing code needs some improvements. Currently the code is entangled in the manager code in odd places (like some of the balancing code is un the Manager.StatusThread class). This change moves the balancing code to its own class w/o making any changes to the code (except fixing two small existing bugs in shouldCleanupMigration()).

Making this change as a first step in making the balancer more independent of other manager code and threads.

[dlmarion]

You should run ScanServerUpgrade11to12TestIT.testScanRefTableCreation with this change locally. It was hanging trying to scan the root and metadata tables because they were not hosted.

[keith-turner]

You should run ScanServerUpgrade11to12TestIT.testScanRefTableCreation with this change locally. It was hanging trying to scan the root and metadata tables because they were not hosted.

I think that is the problem that needs the balancer to run in a separate thread. With this change was just attempting to clean up some tech debt in the way the balancer code is organized before making the threading changes. Hoping we can make a change to spin up three balancer threads and remove balancing from the status thread after this change.

[ctubbsii]

You should run ScanServerUpgrade11to12TestIT.testScanRefTableCreation with this change locally. It was hanging trying to scan the root and metadata tables because they were not hosted.

I think that is the problem that needs the balancer to run in a separate thread. With this change was just attempting to clean up some tech debt in the way the balancer code is organized before making the threading changes. Hoping we can make a change to spin up three balancer threads and remove balancing from the status thread after this change.

I created #5577 to create a new IT to test this, because ScanServerUpgrade11to12TestIT.testScanRefTableCreation should be removed. I think it should be possible to create a reproducer test for that, which does not rely on removing a built-in table.